JP3394612B2 - Non-oxidizing heat retention method for tundish - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tundish non-oxidizing heat retention method when a tundish for continuous casting is repeatedly used.

[0002]

2. Description of the Related Art Molten steel is received from a ladle and distributed to a mold.
Does the tundish have no heating element itself?
In order to ensure the temperature at which casting is possible,
It is necessary to separately heat with a heating means. Also, multiple
A place where continuous casting is performed by exchanging the tundish of the table.
When waiting for a tundish, for example when changing the steel type
Replace it with the one currently used and wait for the next reuse.
But the reuse tundish is also cast
Heating to a visible temperature is required. In any case,
In traditional tundish, generally the tundish is preheated
Preheating by using a gas burner provided on the cover as a heating means
Is done. In that gas burner, for example coke gas
110 to 120% of the theoretical required amount of fuel gas
Send a mixture of air and burn it in the tundish.
The inside of the tundish to 1200-1300 ° C in advance.
It is heating. However, in this case, there is an excess in the combustion gas.
Oxygen from the mixture mixes in the preheated tundish.
In case of continuous reuse, the remaining amount from the previous use (pre-charge)
Steel and residue are oxidized during preheating during the next charge, and FeO
Is generated. This generated FeO reacts with Al in the steel
Al₂O₃Are generated and exist as inclusions in the steel.
As a result, the Al₂O ₃To
This causes quality defects such as baldness and blisters.

Conventionally, the probability of a technique for preventing such a so-called FeO pickup has been sought, and various proposals have been made. For example, in JP-A-4-22567, when the tundish for continuous casting is reused,
By setting the amount of air supplied to the preheating gas burner to 70 to 100% of the theoretical required amount with respect to the amount of supplied gas,
A tundish preheating method is disclosed in which the atmospheric oxygen concentration in the tundish is made lower than in the prior art to suppress the oxidation of residual steel.

Further, in Japanese Patent Laid-Open No. 2-37949,
With the completion of preheating in the tundish, the fuel supply is stopped, and at the same time, the residual fuel in the burner is expelled by the inert gas Ar gas and burned in the preheating cover. There is disclosed a gas replacement technology in a tundish that suppresses the oxidation of residual steel by sending a gas to replace the combustion gas in the tundish with Ar gas in a short time.

[0005]

However, in all of those disclosed in Japanese Patent Laid-Open Nos. 2-37949 and 4-22567, the temperature at which casting can be performed is ensured when the tundish is used. First, it is premised that the fuel gas mixed with air is burned inside the tundish to preheat the inner wall to 1200 to 1300 ° C. Therefore, especially in the case of reused tundish, the problem of oxidation of residual steel during preheating is inevitable. In order to suppress this as much as possible, the technique of Japanese Patent Laid-Open No. 2-37949 proposes a method of blowing an inert gas into the tundish after completion of preheating to purge the combustion gas and residual oxygen to replace the non-oxidizing atmosphere. I am taking it. However, even if the purging method of the inert gas is improved to shorten the time until the completion of gas replacement after preheating a little, the gas purging lowers the inner wall temperature of the tundish and causes heat loss. There is a problem in that it is not possible to prevent even the oxidation of the residue caused by.

On the other hand, in the technique disclosed in Japanese Patent Laid-Open No. 4-22567, the amount of air to the preheating gas burner is set to the theoretical required amount or less to suppress the oxidation of the residual steel without performing the inert gas purging. Therefore, even if the former problem does not occur, it is necessary to set the theoretical air amount of the burner to 50% or less in order to completely prevent oxidation. The problem of combustion occurs, heating costs are high, and safety measures such as explosion proof and CO poisoning countermeasures occur in the treatment of unburned gas.

Therefore, the present invention has been made by paying attention to such problems of the prior art in securing the temperature at which the reusable tundish can be cast, and heat-retaining it by feeding a high temperature inert gas. By omitting the conventional preheating of combustion of fuel gas in the tundish, it is possible to completely prevent the oxidation of residual steel and to provide a tundish non-oxidizing heat retention method that can extend the standby time before reusing the tundish. Is intended.

[0008]

In order to achieve the above object, in the present invention, when reusing a tundish having residual steel on its inner wall, it is heated to at least 850 ° C. or higher by a heating means outside the tundish. The inside of the tundish is kept warm by using an inert gas, and the tundish is used next time.

In addition to the above-mentioned inert gas, a small amount of reducing gas below the explosion limit is introduced into the tundish to make the atmosphere in the tundish non-oxidizing or reducing, thereby completely preventing the oxidation of residual steel. You can also do it. As the heating means for the inert gas, any one of a heat storage type preheater, an electric resistance heater, an induction heater, a plasma torch and the like can be preferably used.

[0010]

The inventors of the present invention, as a measure to solve the above-mentioned conventional problems concerning securing the casting temperature of the reused tundish, reuse the tundish without preheating, that is, do not preheat and oxidize. We have continued to study various experiments to realize the reuse process.

According to experiments conducted by the present inventor, the surface temperature in the tundish during casting is usually almost equal to the molten steel temperature.
Although the temperature rises to about 40 to 1570 ° C., the temperature starts to drop at the same time as the casting ends, and if the temperature is kept as it is, for example, 70 ° C.
11 t after 6 hours in case of t tundish
It falls below 00 ° C and becomes 850 ° C or lower after 14 hours.

At a temperature below 850 ° C., it is difficult to inject the molten steel transferred from the ladle to the tundish into the mold from the nozzle at the bottom of the tundish, even if bubbling with oxygen (so-called enema) is performed from below the nozzle. is there. Also, if the temperature of the tundish during standby falls, the amount of decrease in molten steel temperature when pouring molten steel into the tundish will increase, so in order to secure the molten steel temperature at the beginning of casting, the molten steel temperature during pouring should be high. There is a need to. However, in the latter half of casting, the temperature of the tundish rises, so that the temperature of the molten steel becomes excessively high, which lowers the casting speed and causes breakout. Therefore, at the same time, it was also confirmed by experiments that 850 ° C. can be said to be the lower limit of the temperature when the tundish in standby is reused.

Moreover, the internal pressure of the tundish decreases as the temperature decreases, and when the outside air (oxygen) enters, the oxygen concentration in the tundish increases. It has been known that the oxygen concentration in the standby tundish needs to be 1% or less in order to prevent the oxidation of the residual steel when the tundish is reused. Therefore, in order to prevent oxygen intrusion due to a decrease in the temperature of the tundish during standby without purging the gas in the tundish with an inert gas, the tundish must be sealed almost completely.
The data of the temperature drop of the tundish in the standby state is the value in the closed state.

However, even if it is said that the airtightness is completely sealed, it is practically impossible to make the invasion of air from the outside into the tundish, which continues to shrink with the temperature drop, practically impossible. Achieving complete non-oxidation is difficult. As a countermeasure, it is considered to prevent oxygen from entering from the outside of the tundish by continuous purging with an inert gas (for example, N ₂ gas). According to an experiment conducted by the present inventors on a 70 t tundish in order to examine the possibility, the temperature at the time of standing by while supplying N ₂ gas into the tundish continuously at a rate of 120 Nm ³ / H. It was found that the drop was steeper than that without the previous purge, and decreased to 1100 ° C. in about 3 hours and to 850 ° C. after 8 to 9 hours.

Based on these results, the present inventors have
When reusing the tundish, if the surface temperature inside the tundish is kept at 850 ° C or higher, which is the lower limit of the casting temperature when bubbling is used, with the inert gas heated outside the tundish The inventors have found that it is possible to reuse the tundish while preventing the oxidation without preheating by omitting the preheating by the combustion gas, and completed the present invention.

The means for heating the inert gas is not particularly limited. For example, a regenerative preheater using a regenerator heated by a gas burner as a gas heating source, electric resistance heating or induction heating, or electric heating utilizing a plasma torch. Is preferably used.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a conceptual diagram showing an embodiment of the tundish non-oxidizing heat retention method of the present invention. In FIG.
Reference numeral 1 is a 4-cast tundish (T / D) having a capacity of 70 t. The sliding nozzle and the dipping nozzle at the bottom of the tundish are not shown. Reservoir type preheaters 2 and 2 which are means for heating an inert gas are connected to the openings 1b and 1c of the lid 1a of the tundish 1, respectively. These two heat storage type preheaters 2 and 2 are connected in series via a switching valve 3.

Each heat storage type preheater 2 has a large heat transfer area, for example.
Ceramics or gold in the form of balls or pipes
Heat storage chamber 5 filled with a heat storage body made of metal and its heat storage
Fuel gas combustion chamber 6 for heating the body, and this combustion chamber
6 burner 7 and fuel supply to this burner 7
A line 8 and an air supply line 9 are provided. Switching valve
3 is the inert gas supplied from the inert gas supply line 10.
Gas (eg N ₂, Ar) is one of the heat storage type preheater 2 or
Switch to the other heat storage type preheater 2 in the tundish
Heat storage route from inside the tundish
The gas taken out through the preheater 2 is fed to the exhaust fan 11.
After that, take out from the tundish
The stored gas on one side of the regenerative preheater 2 and on the other side of the regenerative preheat
The route for recycling into the tundish via vessel 2
It has a function to switch freely.

Using the apparatus shown in FIG.
N₂Non-oxidative heating experiment of tundish 1 using gas
Was carried out as follows. (1) Lid 1 on the tundish 1 after being used for the first time
Attach a and turn off the two heat storage type preheaters 2 and 2 alternately.
High temperature heating N that was used by replacing and was heated to 1300 ° C ₂Gas
Tundish 1 is continuously supplied to the tundish.
Experiment to keep heat inside the chamber: At this time, the bar of the heat storage type preheater 2
Fuel gas from the fuel supply line 8
Combustion in the combustion chamber 6 while supplying air from the in 9
70 × 10^FourGenerate heat of Kcal / Hr
The heat storage body of the heat storage chamber 5 was heated. Then burner 7
1800 Nm from the outside via the switching valve 3³/ Hr
At the flow rate of N₂Send gas and pass through the heated heat storage body 1
This is heated to a temperature of 300 ° C or higher and heated at high temperature.₂Gas
I sent it into Tundish 1. One heat storage type preheater
2 for N₂The other heat storage type preheater 2 is being used for heating gas.
The heat storage body is heated.

In this heat storage body heating step, the burned gas in the combustion chamber 6 is sucked and exhausted by the exhaust fan 11 through the heat storage chamber 5 and the switching valve 3. In this experiment, the temperature is 200-30
Combustion exhaust gas lowered to 0 ° C. and N ₂ gas sucked from the tundish 1600 to 2000 Nm ³ / Hr
The regenerator was heated while forcing exhaustion. The high-temperature heated N ₂ gas sent into the tundish 1 blows out through the clearance of the lid 1a of the tundish, the openings 1b, 1c, etc. and leaks, but the internal pressure in the tundish 1 is kept slightly higher than the external pressure. This prevents outside air from entering the tundish. Also, the above-mentioned external supply 1
By recycling 20 to 60% of the amount of N ₂ gas of 800 Nm ³ / Hr, temperature control in the baking chamber 5 and recovery of N ₂ gas waste heat are performed.

This N ₂ gas heating is alternately repeated every 60 seconds by using two heat storage type preheaters 2 and 2 to obtain 13
A high temperature N ₂ gas of 00 ° C. or higher is continuously supplied into the tundish 1 to keep the inner surface of the tundish 1 at a temperature of 850 ° C. or higher while keeping the tundish in a non-oxidizing atmosphere for reuse. I was able to make Tundish 1 stand by until the start.

When switching the heat storage type preheater 2,
Even after extinguishing the burner 7 of one of the heat storage type preheaters 2, a part of the N ₂ gas in the tundish 1 is continued by continuing the forced exhaust in the combustion chamber 6 by the exhaust fan 11 for a certain period of time. Is discharged from the high temperature N ₂ gas insertion pipe 2a of the heat storage type preheater 2 through the combustion chamber 6, the heat storage chamber 5, and the switching valve 3, so that the combustion chamber 6, the heat storage chamber 5, and the switching valve 3 of the heat storage type preheater 2 are discharged. The combustion gas remaining inside can be purged and replaced with an inert gas. Thus, by preventing the residual combustion gas generated in the initial stage of the switching operation from being mixed into the tundish, the inside of the tundish 1 can be completely maintained in the non-oxidizing atmosphere.

(2) Effect of prolonging the standby time of the tundish that has been kept non-oxidized: Next, using the apparatus shown in FIG. 1, the tundish having an inner surface temperature of 1300 ° C. or higher initially has a temperature of 850 ° C. The effect of extending the standby time of the tundish when the non-oxidative heat was kept while continuously feeding the heated N ₂ gas was determined in comparison with the conventional one.

The results are shown in the graph of FIG. The current curve with purging shows that the tundish with the inner surface temperature of 1350 ° C. is capped, and N ₂ gas at room temperature is supplied at a flow rate of 120 Nm ³ / H to purge the inside of the tundish while standing by. The transition of the surface temperature is shown. The waiting time until the lower limit temperature of casting is 850 ° C is 8 to 9
It's time.

The current curve without purging shows that the inner surface temperature is 13
In the case where the tundish at 50 ° C. is capped and is kept on standby without purging with an inert gas, the standby time is extended to about 14 hours. On the other hand, according to the method of the present invention, by keeping the heat while supplying the inert gas at 1300 ° C. to the tundish having the inner surface temperature of 1350 ° C., the standby time can be greatly extended to 24 hours, and the temperature can be increased continuously. The number could be increased.

(3) Non-oxidative heat retention with introduction of a trace amount of reducing gas: In the apparatus of FIG.
Not shown connects the reducing gas supply line, together with an inert gas H _2, CO, reducing gas such as CH ₄ (LPG
T may be used instead of T) 1)
The atmosphere in the tundish was made reducible and heat was maintained by introducing a trace amount into it. Here, the small amount is an amount that can prevent an explosion when the reducing gas leaks to the outside of the tundish, that is, an amount within the explosion limit of the reducing gas, for example, in the case of H ₂ . The inside of the tundish 1 is kept heat by mixing an inert gas in an amount of 4% or less and 12.5% or less in the case of CO.

As a result, the atmosphere in the tundish becomes a reducing atmosphere, there is no danger of explosion at the time of leakage, and the oxidation of the residual steel can be prevented more completely. FIG. 3 shows still another embodiment of the heating means of an inert gas for tundish non-oxidative heat retention. This uses a non-transfer type plasma torch 20 as a heating means for the inert gas. This type of plasma torch 20
Has the anode 22 on the torch itself together with the cathode 21, and the inert gas flow supplied to the torch via the cathode 21 is turned into plasma by the discharge of both electrodes 21 and 22, and the high temperature plasma 23 thus obtained By tundish 1
The inner wall surface of the. Ar, N as plasma gas
It is also possible to use ₂ etc. and use HN gas together.

In general plasma jet heating, a plasma temperature of 3000 to 10000 ° C. is used, but in the present invention, the temperature is reduced to 2000 ° C. or less by causing the atmospheric gas in the tundish to be entrained in the plasma jet. It is used as a lowered high-temperature jet gas and is heated at 1000 to 1300 ° C. in a non-oxidizing atmosphere. That is, the inert gas sent into the tundish 1 is made into plasma by the plasma torch 20 attached to the lid 1a of the tundish 1 and is blown to the bottom of the tundish 1. The heat transfer during this heating takes the form of convective transfer from the hot gas stream and radiative heat transfer from the heated tundish bottom surface to the other surface.

However, in the case of plasma jet heating,
In order to reduce the running cost, the tundish was heated for the time necessary to secure the inner surface temperature of the tundish at 1300 ° C. before being reused, and the pre-heat standby was set during the other standby time. In Fig. 4, the thickness of the heat insulating material is 30m
The result of carrying out the non-oxidation heat retention experiment of the tundish by the plasma torch 20 in the case of casting using the adiabatic tundish of m is shown.

When the tundish, which had been at a temperature of 1570 ° C. during casting, was allowed to stand by without preheating, the internal surface temperature of the tundish dropped to 1100 ° C. or less after a waiting time of 7 hours.
Then, the non-oxidizing heating in the tundish was started by the N ₂ gas plasma jet using the plasma torch 20, and 4
Surface temperature in the tundish after 1 hour is 1300 ° C
Reachable for reuse. The total waiting time is 11 hours, during which one charge 4 with other tundish.
We were able to perform 16 minutes of 0 minute casting.

In the above embodiment, a plasma torch was used as a means for electrically heating the inert gas in the tundish non-oxidizing heat retention method. You may use a container.

[0032]

As described above, according to the first aspect of the present invention.
In accordance with the invention of at least 8 reused tundish.
Since the heat is retained by using the inert gas heated to 50 ° C or higher, the conventional preheating such as combustion of the fuel gas in the tundish can be omitted, and as a result, the oxidation of the residual steel can be completely prevented and This has the effect of significantly extending the waiting time before reusing the dish.

According to the second aspect of the invention, since a small amount of reducing gas below the explosion limit is introduced into the tundish in addition to the inert gas, the atmosphere in the tundish is reduced. As a result, it is possible to more completely prevent the oxidation of the residual steel. Further, according to the invention of claim 3, since the heat storage type preheater is used as the heating means for the inert gas, the conventional gas heating equipment is utilized and the non-oxidative heat retention of the tundish without the residual steel oxidation is realized. Has the effect of doing.

Further, according to the invention of claim 4, since the electric heating system such as the electric resistance heater, the induction heater or the plasma torch is used as the heating means for the inert gas, the thermal efficiency of the inert gas heating is improved by the gas heating. The effect that it can be improved compared to.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing one embodiment of the tundish non-oxidizing heat retention method of the present invention.

FIG. 2 is a graph showing the effect of extending the standby time of the tundish when the tundish is heat-treated without oxidation by the method of FIG.

FIG. 3 is a conceptual diagram showing still another embodiment of the tundish non-oxidizing heat retention of the present invention.

FIG. 4 is a graph showing changes in the tundish temperature in the tundish non-oxidizing heat retention method shown in FIG.

[Explanation of symbols]

1 tundish 2 Heating means (heat storage type preheater) 20 Heating means (plasma torch)

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Junichi Hasunuma 1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama Prefecture (no address) Inside the Mizushima Steel Works, Kawasaki Steel Co., Ltd. (72) Takemi Yamamoto 1-shima-shima Kawasaki, Kurashiki-shi, Okayama Chome (no address) In Kawashima Steel Co., Ltd. Mizushima Steel Works (56) Reference JP-A-6-281350 (JP, A) JP-A-4-22568 (JP, A) JP-A-8-159664 (JP, A) ) JP 4-162955 (JP, A) JP 4-143047 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/10 310 B22D 41/015

Claims (4)

(57) [Claims] 1. When reusing a tundish having residual steel on its inner wall, the inside of the tundish is kept warm by using an inert gas heated to at least 850 ° C. or higher by a heating means outside the tundish, and is then used next time. A tundish non-oxidative heat retention method characterized by providing. 2. The tundish atmosphere is made non-oxidizing or reducing atmosphere by introducing a small amount of reducing gas below the explosion limit into the tundish in addition to the inert gas. Non-oxidative heat retention method for tundish. 3. The tundish non-oxidizing heat retention method according to claim 1, wherein the heating means is a heat storage type preheater. 4. The heating means is an electric resistance heater, an induction heater or a plasma torch.
Non-oxidative heat retention method of the described tundish.